Secreted and transmembrane proteins play an essential role in intercellular communication during the development of multicellular organisms. Because only a small number of these genes have been characterized, we developed a screen for genes encoding extracellular proteins that are differentially expressed during Drosophila embryogenesis. Our approach utilizes a new method for screening large numbers of cDNAs by whole-embryo in situ hybridization. The cDNA library for the screen was prepared from rough endoplasmic reticulum-bound mRNA and is therefore enriched in clones encoding membrane and secreted proteins. To increase the prevalence of rare cDNAs in the library, the library was normalized using a method based on cDNA hybridization to genomic DNA-coated beads. In total, 2,518 individual cDNAs from the normalized library were screened by in situ hybridization, and 917 of these cDNAs represent genes differentially expressed during embryonic development. Sequence analysis of 1,001 cDNAs indicated that 811 represent genes not previously described in Drosophila. Expression pattern photographs and partial DNA sequences have been assembled in a database publicly available at the Berkeley Drosophila Genome Project website (http://fruitf ly.berkeley.edu). The identification of a large number of genes encoding proteins involved in cell-cell contact and signaling will advance our knowledge of the mechanisms by which multicellular organisms and their specialized organs develop.

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A critical step in Drosophila dorsoventral patterning is the movement of gurken mRNA from the anterior cortex of the oocyte to the oocyte's anterodorsal corner at stage 8 of oogenesis. Such movement is dependent on fs(1)K10. It has been proposed that fs(1)K10 mediates gurken mRNA movement by down-regulating gurken mRNA levels, thus ensuring that gurken mRNA does not saturate its receptors located in the oocyte's anterodorsal corner. In contradiction to this model, we show here--both genetically and immunocytochemically--that GRK protein levels are lower in the anterodorsal region of fs(1)K10 mutant oocytes than in the anterodorsal region of fs(1)K10+ oocytes. From this and other data, we propose a more direct role for fs(1)K10 in the gurken mRNA localization process.